This invention relates to a solid electrolyte capacitor process, and more particularly to a process for the deposition of a solderable metal layer onto the surface of a solid electrolyte capacitor by chemical means.
It is conventional in the prior art to produce a solid electrolyte capacitor by anodizing a valve-metal pellet, coating the dielectric oxide thus formed with a solid electrolyte, applying a layer of graphite or colloidal carbon as a cathode contact to the solid electrolyte, and applying a layer of silver paint or silver epoxy over the carbon. It is also conventional to solder a cathode lead to the silver layer after the layer is cured.
Unfortunately, while the silver layer is easy to apply, it is expensive. It also migrates during capacitor operation into the solder used to apply the cathode lead to the detriment of continued capacitor operation. Using a paint or epoxy containing a solderable metal other than silver, e.g., copper, has not been satisfactory as it has proved difficult to solder the cathode leads to such cured paints.
Other solderable materials, namely copper, have been applied as sprayed layers or electroplated onto the carbon surface. These techniques are relatively expensive, which is why the silver paints have enjoyed such widespread usage. Attempts to replace part of the silver in the paint with another metal, e.g., copper, have improved but have not solved the silver migration problem.
Electroless plating of metals has been used in the manufacture of solid-electrolyte capacitors, but to replace the solid electrolyte rather than plate over it. By and large, the solutions used to prepare or catalyze the surface to be plated are harmful to the solid-electrolyte layer, particularly when it is manganese dioxide.
Therefore, it would be advantageous to be able to replace the silver layer with a different solderable metal using a simple process to do so and without damaging the solid-electrolyte layer.